Place the pistachios, coffee granules and sugar in a food processor and process until ground. Add the butter and flour and pulse until just combined. Roll out thinly between baking paper to form a 30 × 20 cm (12 × 8 inch) rectangle. Place on a baking tray and rest in the fridge for at least 30 minutes.
Preheat the oven to 160°C (315°F/Gas 2–3). Remove the top sheet of baking paper and bake for 12 minutes or until golden. Remove from the oven and allow to cool on the tray. Using a mortar and pestle, crush the coffee and pistachio biscuit into fine crumbs.